As a preliminary step in the manufacture of metal cans from a metal sheet, one or more layers of coating material are applied to the sheet. These layers may provide functional protective features and may also be applied for decorative purposes. Examples of such coating material are sizes, pigmented coatings, inks, lacquers and varnishes. For example, where a metal sheet is to be used in the manufacture of bodies for three piece metal food cans, a layer of lacquer may be applied to one surface of the sheet. By way of another example, where a metal sheet is to be used in the manufacture of bodies for aerosol cans, a layer of lacquer may be applied to one surface of the sheet and layers of size, white coating, inks, and varnish may be applied, in turn, to the other surface. Where a metal sheet is being used in the manufacture of can bodies, after it has received the required number of layers of coating material, small rectangular workpieces are cut from the sheet and these are then formed into can bodies. Where a sheet is used in the manufacture of other components, for example, ends for can bodies, workpieces of suitable shape and size are cut from the metal sheet and then formed into the desired components.
After a layer of coating material has been applied to a metal sheet, the coating material is cured by passing the sheet through an oven. A conventional oven for metal sheets comprises a heating section, a cooling section and a wicket conveyor. A wicket conveyor comprises a series of wickets which are mounted at spaced intervals on endless chains. Each wicket is arranged to carry a sheet through the oven in a substantially vertical position with the coated and wet surface of the sheet out of contact with the wicket. When an oven is in operation, the wicket conveyor carries sheets in turn firstly through the heating section and then through the cooling section. In the heating section, heated air is passed over the sheets. Typically, in the heating section the temperature of each sheet is raised to a temperature in the range of 120.degree. C. to 230.degree. C., the transit times of the sheets through the heating and cooling sections are 10 to 20 minutes and 3 to 5 minutes, respectively, and 5000 sheets an hour enter the oven. Thus, it is normal for about 1600 sheets to be present in an oven. The temperature and transit times may be varied in accordance with the specific coating material being used.
There are various problems associated with the conventional oven. During operation, volatile components from the coatings condense on the wickets. In other circumstances, the wickets may corrode. Thus, the wickets are a source of dirt. If such dirt is carried by air currents in the oven and transferred to a layer of coating material while it is still wet, the coating material is spoilt. In order to reduce the presence of dirt, the wickets are cleaned periodically and this results in a loss of production time. As a metal sheet is carried through the heating section by a wicket, there are periods when the wicket and metal sheet are at different temperatures. Consequently, heat transfer between the wicket and the metal sheet can interfere with the curing process and this leads to marks in the cured layer of coating material. The wickets are prone to vibration and the vibration together with sheet movements induced by air currents can cause abrasion damage on the sides of the sheets resting on the wickets. Occasionally, uneven heating causes a sheet to buckle. Such buckling together with movement induced by air currents can cause contact between a sheet and a neighboring wicket. Such contact causes damage to the coating. Because one surface of a metal sheet rests against a wicket as it passes through the oven, it is possible to cure only a single surface of the sheet during each passage. While the arrangement of coated sheets supported on their lower edges on wickets has proved satisfactory for many years (apart from the problems just mentioned), there is now a requirement to coat thinner metal sheets having thicknesses less than 0.2 mm. Such sheets tend to bend when supported on their edges and such bending could cause contact between a sheet and a neighboring wicket.